ZHCSJO8 May 2019 OPA818
ADVANCE INFORMATION for pre-production products; subject to change without notice.
Often two primary considerations for TIA applications are maximizing TIA closed-loop bandwidth and minimizing the total output noise to maximize Signal-to-Noise Ratio (SNR). The total input capacitance (CTOT) of TIA circuit causes a zero in the noise gain in combination with the transimpedance gain (feedback resistor, RF) at frequency 1/(2πRFCTOT). For a fixed RF, this zero is at a lower frequency for higher CTOT thus increasing the noise gain at lower frequency resulting in lower equivalent closed-loop bandwidth and higher total output noise compared to a lower CTOT. By choosing an amplifier like OPA818 that features a low input capacitance (2.4 pF combined common-mode and differential) for TIA application, the system designer can realize high closed-loop bandwidth at low total output noise or have the flexibility to choose a photodiode with relatively higher capacitance. The CTOT includes the input capacitance of the amplifier, the photodiode capacitance, and the PCB parasitic capacitance at the inverting input.